1. Field of the Invention
The present invention relates to a method of controlling slag coating of a steel converter for attaining improved refractory life. More particularly, the present invention is concerned with a coating control method capable of preventing coating slag from flowing out of the converter during blowing.
2. Description of the Related Art
In recent years, a trend toward increasing steel converter capacities has arisen. At the same time, steel converters have been required to operate under increasingly severe conditions. Concurrently, a demand for improved production efficiency and reduced production cost of steel has increased. Consequently, prolonging converter furnace life has become a strong priority. Various methods have been proposed to lengthen converter service life.
One such technique involves coating converter with slag to extend refractories life. This technique may be further divided into two general methods.
One such method involves elevating CaO and MgO content during blowing to chemically suppress melting of the refractories.
The other method, which is known as "slag coating," makes use of residual slag that remains after blowing. More specifically, a slag solidifying agent, brick chips and other ingredients are mixed with the residual slag to form a "coated slag." After a blowing, this coated slag is welded onto the surfaces of refractories bricks on the side and/or bottom walls of the converter by spraying or by tilting the converter. The welded layer of the coated slag will be referred to as the "coating layer."
In the first-mentioned method, melting of the refractories is suppressed by a chemical effect which inhibits dissolution of components of the refractories. Conversely, the second-mentioned method reduces thermal load imposed on the refractories so as to suppress thermal spalling. In extending converter refractory life, it is important to prevent rapid damage and wear of the bricks through effective suppression of thermal spalling.
Various methods for suppressing thermal spalling have been proposed. Japanese Patent Laid-Open No. 61-157610 proposes a method in which the inner surfaces of a converter are coated with coated slag which is formed by adding a slag solidifying agent into residual slag, while means are provided for forcibly cooling the coating layer. When the blowing for the next charge of steel is conducted for a long time at high temperature, however, the slag itself is heated to a high temperature such that the coating layer cannot be retained due to melting and flowing down of the slag.
Japanese Patent Laid-Open No. 61-56223 discloses a method in which slag is left in the converter after each steel discharge and is blown by a gas from an upper blowing lance such that the slag is scattered--deposited on the inner surfaces of the converter. When the slag viscosity is low enough to permit scattering, the coating layer cannot be retained due to melting and flowing down of the slag. On the contrary, when the slag viscosity is high, the slag does not fly freely and the converter inner surfaces are not adequately coated.
Japanese Patent Publication No. 62-24490 discloses a method which utilizes a converter that does not employ dephosphorization and desulfurization. The slag composition at the end of the blowing is controlled to meet the conditions of CaO/SiO.sub.2 =1.6 to 2.5, MgO/CaO&gt;0.25 and SiO.sub.2 /(CaO+MgO+SiO.sub.2).gtoreq.0.25. The deposition of the slag is believed to be enhanced by the condition wherein SiO.sub.2 /(CaO+MgO+SiO.sub.2).gtoreq.0.25. Unfortunately, this method does not allow the coating layer to be retained after the end of the subsequent blowing cycle.
Japanese patent Publication No. 2-2992 discloses a method in which chromium ore is charged before oxygen blowing in a high-chromium melting converter to maintain a slag composition containing 30 to 50% Cr.sub.2 O.sub.3. This method seeks to control slag composition during blowing. Increasing the fluidity of the slag impairs coating but improves slag formation efficiency. Conversely, when slag fluidity is reduced to improve coating characteristics, slag formation efficiency is undesirably lowered.
Japanese Patent Publication No. 62-13407 discloses a method in which a powdered refractories, mainly composed of MgO, is blown by flame gunning onto slag remaining in a converter so as to coat the inner surfaces of the converter with the mixture of the slag and the powdered refractories. This method requires an impractically long time for the blowing and, hence, cannot be used where the tapping interval is short.
Japanese Patent Publication No. 61-59364 discloses a method in which a basic refractories in the form of bulk or aggregates of 100 to 200 mm is charged into the slag remaining in the converter so that the slag is solidified by being kept stationary for 15 minutes. This method is suitable for repairing portions of the converter damaged by spalling, but cannot be used for repairing of the trunnion side of the converter.
Japanese Patent Laid-Open No. 2-111810 discloses a method in which, in order to prevent the slag floating on the steel melt surface from being discharged together with the steel, the solid volume fraction of the slag is maintained at 30% or greater. This method prevents quality deterioration of the discharged steel due to presence of slag. However, the tapping operation tends to be impeded when the fluidity of the slag is excessively decreased. This method, therefore, cannot be applied to slag coating methods that require the coating to sustain direct contact with the flowing steel melt during a long blowing period.